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I read a fascinating review/article that mentioned in passing this plot point. I can't find it right now and I'll edit this post when I do. (AHA FOUND IT). The author of this review put forward a theory which blew my mind.

We see in the movie that not just "Loopers" and people sent to disappear are sent to the past. We saw that Abe was sent back from the future to hire Loopers and keep an eye on that end of things.

In the end Sara convinces Cid that she is his mother and it was her "sister" who was killed accidentally by his tantrum. What if Sara was sent from the future to the year 2044 so that Cid would have a stable childhood and not grow to become the Rainmaker?

Here is a quote from the article (which the writer/director Rian Johnson retweeted so probably thinks is an interesting theory if not what he was planning when writing it):

Sara sits down outside of her house, and pantomimes taking a drag from a cigarette, while spinning her free hand. She does the exact same thing later, just after sleeping with young Joe. She’s smoking while using her TK powers to float her lighter. THIS event is the final thing that ensures that Joe cares enough about the two of them to kill himself to prevent old Joe from turning Cid into the Rainman. It should also be noted that Cid - who is a GENIUS - also says candidly that she’s not his mom.

This means she’s could actually someone looping back to care for Cid, after he accidentally killed his real mother in a psychic tantrum. This would explain how Cid originally came to be motherless and turn into Rainman without old Joe’s involvement. This would also explain how she knows about Loopers, why she does’t shoot Joe when she sees him, why she bears a striking resemblance to Joe’s hookup Suzie so as to attract him, and how she would know what to do in order to ensure that Cid doesn’t become Rainman.

They're known about in various circles, and may just be kind of that thing you don't discuss, like the mob in general. She may have just heard about it in passing, which could be part of the reason why she chose to move so far away from city life.

Its implied that she was a hooker, and that she probably had Loopers as clients.

From a mere writing standpoint it could just have been a shortcut, so that Joe doesn't get a hard time explaining to her all the things she surely wouldn't believe, which wouldn't bring the story much further (Ok, maybe it could have opened other interresting conflicts and sub-stories, but well).

And in fact when he started "time travel is not possible, but in thirty years..." I found myself thinking nerved "Whoa man, stop it, she's never gonna buy that" only to be relieved when she said "You're a looper", thinking "thanks, not a hard long mistrustful explanation session".

I'm not saying that's the only reason (or that everybody would have felt like me and explaining it to her couldn't have been a good story arc, too) and there is no in-story explanation. But seeing that not everything in Looper was that thought out well, it's a possibility.

Thank you for your interest in this question.
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